Method of measurement, an inspection apparatus and a lithographic apparatus

ABSTRACT

An inspection system is arranged to measure an overlay error by projecting a plurality of radiation beams, differing in wavelength and/or polarization, onto two targets. A first radiation beam is projected onto a first target and the reflected radiation A 1+  is detected. The first target comprises two gratings having a bias +d with respect to each other. The first radiation beam is also projected on to a second target, which comprises two gratings having a bias −d with respect to each other, and the reflected radiation A 1−  is detected. A second radiation beam, having a different wavelength and/or polarization from the first radiation beam, is projected onto the first target and reflected radiation A 2+  is detected and projected onto the second target and reflected radiation A 2−  is detected. Detected radiations A 1+ , A 1− , A 2+ , and A 2−  is used to determine the overlay error.

FIELD

The present invention relates to a method of inspection usable, forexample, in the manufacture of devices by a lithographic technique andto a method of manufacturing devices using a lithographic technique.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to determine one or more properties of the substrate, such asthe overlay between a resist pattern and an underlying processedpattern, a beam is reflected off the surface of the substrate, forexample at an alignment target, and an image is created on, for example,a camera of the reflected beam. By comparing one or more properties ofthe beam before and after it has been reflected off the substrate, aproperty of the substrate can be determined. This can be done, forexample, by comparing the reflected beam with data stored in a libraryof known measurements associated with one or more known substrateproperties.

SUMMARY

Such a system of illuminating a target and collecting data from thereflected radiation is often used to illuminate a plurality ofsuperimposed patterns, for example a plurality of gratings. The secondgrating has a predetermined bias compared to the first grating. Byanalyzing the characteristics of the reflected radiation it is possibleto determine the overlay error, OV, between the gratings. This isachieved by introducing a known shift, d, between gratings in differentlayers. However, the detected overlay accuracy is dependent on both theoverlay accuracy itself and asymmetry due to sensor asymmetry resultingfrom, for example, radiation scattering. A method of reducing the effectof sensor asymmetry includes rotating the substrate but this takes timeand may result in a significant loss of throughput. An alternative is touse a reference target but this also may require significant time andspace on the substrate since many different targets are required.

It is desirable to, for example, provide an alternative, simple methodof reducing the effect of sensor asymmetry.

According to an aspect of the invention, there is provided a method ofmeasuring the overlay error of a substrate, comprising:

projecting a first beam of radiation onto a first target of thesubstrate, the first target comprising at least two superimposedpatterns having a bias of +d between a first of the patterns arranged inor on a first layer and a second of the patterns arranged in or on asecond layer, and measuring the asymmetry of that first beam ofradiation reflected from the substrate that is indicative of a propertyof the substrate and generating a first signal indicative of themeasured asymmetry;

projecting the first beam of radiation onto a second target of thesubstrate, the second target comprising at least two superimposedpatterns having a bias of −d between a first of the patterns arranged inor on a first layer and a second of the patterns arranged in or on asecond layer, and measuring the asymmetry of that first beam ofradiation reflected from the substrate that is indicative of a propertyof the substrate and generating a second signal indicative of themeasured asymmetry;

projecting a second beam of radiation onto the first target andmeasuring the asymmetry of that second beam of radiation reflected fromthe substrate that is indicative of a property of the substrate andgenerating a third signal indicative of the measured asymmetry;

projecting the second beam of radiation onto the second target andmeasuring the asymmetry of that second beam of radiation reflected fromthe substrate that is indicative of a property of the substrate andgenerating a fourth signal indicative of the measured asymmetry,

wherein the first and second beams have different polarizations, ordifferent wavelengths, or both, and the overlay error is determined onthe basis of the first, second, third and fourth signals.

According to a further aspect of the invention there is provided aninspection apparatus configured to measure a property of a substrate,the apparatus comprising:

a radiation projector configured to project radiation onto thesubstrate;

a detector configured to measure asymmetry of radiation reflected fromthe substrate; and

a processor configured to calculate an overlay error on the basis of theasymmetry, measured by the detector, of radiation of a plurality ofwavelengths, or a plurality of polarizations, or both, reflected fromthe substrate.

According to a further aspect of the invention there is provided alithographic apparatus, comprising:

an illuminator configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning devicebeing capable of imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate; and

an inspection apparatus configured to measure a property of a substrate,the inspection apparatus comprising:

-   -   a radiation projector configured to project radiation onto the        substrate;    -   a detector configured to measure asymmetry of radiation        reflected from the substrate; and    -   a processor configured to calculate an overlay error on the        basis of the asymmetry, measured by the detector, of radiation        of a plurality of wavelengths, or a plurality of polarizations,        or both, reflected from the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 a depicts a lithographic apparatus;

FIG. 1 b depicts a lithographic cell or cluster;

FIG. 2 depicts a scatterometer;

FIG. 3 depicts the general operating principle of measuring an angleresolved spectrum in the pupil plane of a high-NA lens;

FIG. 4 depicts a substrate used in conjunction with an embodiment of theinvention;

FIG. 5 depicts a detailed view of the substrate depicted in FIG. 4; and

FIG. 6 depicts first and second diffraction orders.

DETAILED DESCRIPTION

FIG. 1 a schematically depicts a lithographic apparatus. The apparatuscomprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or EUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PLconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index,. e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1 a, the illuminator IL receives a radiation beam froma radiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PL,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1 a) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PL. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 1 b, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell(lithographic cell) or cluster, which also includes apparatus to performone or more pre- and post-exposure processes on a substrate.Conventionally these include one or more spin coaters SC to depositresist layers, one or more developers DE to develop exposed resist, oneor more chill plates CH and one or more bake plates BK. A substratehandler, or robot, RO picks up substrates from input/output ports I/O1,I/O2, moves them between the different process devices and delivers thento the loading bay LB of the lithographic apparatus. These devices,which are often collectively referred to as the track, are under thecontrol of a track control unit TCU which is itself controlled by thesupervisory control system SCS, which also controls the lithographicapparatus. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that a substrate that is exposed by the lithographic apparatusis exposed consistently for each layer of resist, there are one or moreproperties of the substrate that should be measured to determine whetherthere are changes in alignment, rotation, etc., that must be compensatedfor by the lithographic apparatus. A separate inspection apparatus isused to determine the one or more properties of the substrate, and inparticular, how the one or more properties of different substrates ordifferent layers of the same substrate vary from layer to layer.

A property of the surface of a substrate W may be determined using asensor such as a scatterometer such as that depicted in FIG. 2. Thescatterometer comprises a broadband (white light) radiation projector 2which projects radiation onto a substrate W. The reflected radiation ispassed to a spectrometer detector 4, which measures a spectrum 10(intensity as a function of wavelength) of the specular reflectedradiation. From this data, the structure or profile giving rise to thedetected spectrum may be reconstructed, e.g. by Rigorous Coupled WaveAnalysis and non-linear regression or by comparison with a library ofsimulated spectra as shown at the bottom of FIG. 2. In general, for thereconstruction the general form of the structure is known and someparameters are assumed from knowledge of the process by which thestructure was made, leaving only a few parameters of the structure to bedetermined from the scatterometry data. The radiation source 2 may bepart of the scatterometer or may simply be conduit of radiation from anoutside radiation generator.

The scatterometer may be a normal-incidence scatterometer or anoblique-incidence scatterometer. Variants of scatterometry may also beused in which the reflection is measured at a range of angles of asingle wavelength, rather than the reflection at a single angle of arange of wavelengths.

A scatterometer configured to measure one or more properties of asubstrate may measure, in the pupil plane 11 of a high numericalaperture lens 15, a property of an angle-resolved spectrum reflectedfrom the substrate surface W at a plurality of angles and wavelengths asshown in FIG. 3. Such a scatterometer may comprise a radiation projector2 to project radiation onto the substrate and a detector 14 configuredto detect the reflected spectrum. The pupil plane is the plane in whichthe radial position of radiation defines the angle of incidence and theangular position defines the azimuth angle of the radiation. Thedetector 14 is placed in the pupil plane of the high numerical aperturelens 15. The numerical aperture may be high, e.g., in an embodiment, atleast 0.9 or at least 0.95. An immersion scatterometer may even have alens with a numerical aperture over 1.

An angle-resolved scatterometer may measure the intensity of scatteredradiation. A scatterometer may also or in addition allow severalwavelengths to be measured simultaneously at a range of angles. Aproperty measured by the scatterometer for different wavelengths andangles may be the intensity of transverse magnetic- and transverseelectric-polarized radiation and the phase difference between thetransverse magnetic- and transverse electric-polarized radiation.

Using a broadband radiation source (i.e. one with a wide range ofradiation frequencies or wavelengths—and therefore of colors) ispossible, which gives a large etendue, allowing the mixing of multiplewavelengths. The plurality of wavelengths in the broadband preferablyeach has a bandwidth of, say, *8 and a spacing, therefore, of at least2*8 (i.e. twice the wavelength). Several “sources” of radiation may bedifferent portions of an extended radiation source which have been splitusing, e.g., fiber bundles. In this way, angle resolved scatter spectramay be measured at multiple wavelengths in parallel. A 3-D spectrum(wavelength and two different angles) may be measured, which containsmore information than a 2-D spectrum. This allows more information to bemeasured which increases metrology process robustness. This is describedin more detail in European patent application publication EP1,628,164A.

A scatterometer that may be used with an embodiment of the presentinvention is shown in FIG. 3. The radiation of the radiation projector 2is focused using lens system 12 through interference filter 13 andpolarizer 17, reflected by partially reflective surface 16 and isfocused onto substrate W via a microscope objective lens 15. Theradiation is then transmitted through partially reflective surface 16onto a CCD detector in the back projected pupil plane 11 in order tohave the scatter spectrum detected. The pupil plane 11 is at the focallength of the lens system 15. A detector and high aperture lens areplaced at the pupil plane. The pupil plane may be re-imaged withauxiliary optics since the pupil plane of a high-NA lens is usuallylocated inside the lens. The radiation source 2 may be part of thescatterometer or may simply be conduit of radiation from an outsideradiation generator.

A reference beam is often used for example to measure the intensity ofthe incident radiation. When the radiation beam is incident on thepartially reflective surface 16 part of it is transmitted through thepartially reflective surface towards a reference mirror 14. Thereference beam is then projected onto a different part of the same CCDdetector 18.

The pupil plane of the reflected radiation is imaged on the CCD detectorwith an integration time of, for example, 40 milliseconds per frame. Inthis way, a two-dimensional angular scatter spectrum of a substratetarget is imaged on the detector. The detector may be, for example, anarray of CCD or CMOS sensors.

A set of interference filters 13 is available to select a wavelength ofinterest in the range of, say, 405-790 nm or even lower, such as 200-300nm. The interference filter may be tunable rather than comprising a setof different filters. A grating could be used instead of an interferencefilter.

The substrate W may comprise a grating which is printed such that afterdevelopment, the bars are formed of solid resist lines. The bars mayalternatively be etched into the substrate.

When radiation is reflected by the grating of the substrate W, thetransmission of first and second orders, depicted in FIG. 6 are givenby:

T ⁻¹ =T ₀ −ΔT

T ₊₁ =T ₀ +ΔT

where T₀ is the average sensor transmission and ΔT is the sensorasymmetry.

Imperfections in the optics and scattering due to the debris particlescause asymmetric scattering of the radiation which is not very sensitiveto polarization or wavelength. In contrast the asymmetric scattering ofradiation due to overlay error is highly sensitive to polarization andwavelength.

To measure the overlay error, OV, between different exposed layers ofthe substrate, grating patterns, in the respective different layers,having a bias, d, with respect to each other are exposed. Theintensities of the +1^(st) and −1^(st) order reflected radiation forsmall overlay errors can be approximated as:

I ₊₁=(T ₀ +ΔT)[B ₀ +B ₁(OV+d)]

I ⁻¹=(T ₀ −ΔT)[B ₀ −B ₁(OV+d)]

where B₀ is the intensity of the first orders without overlay error andB₁ is a proportionality factor that describes the sensitivity of theintensity for small overlay errors. T₀ and ΔT are the average sensortransmission and the sensor asymmetry respectively. Further detailsabout this can be found in European patent application publicationEP1,628,164. The asymmetry A between the +1^(st) and −1^(st) orderintensities is:

$\begin{matrix}{A_{+} = \frac{I_{+ 1} - I_{- 1}}{I_{+ 1} + I_{- 1}}} \\{= \frac{{K\left( {{OV} + d} \right)} + {\Delta \; A}}{1 + {\Delta \; {{AK}\left( {{OV} + d} \right)}}}} \\{\cong {{K\left( {{OV} + d} \right)} + {\Delta \; A}}}\end{matrix}$ where $K = \frac{B_{1}}{B_{0}}$${\Delta \; A} = \frac{\Delta \; T}{T_{0}}$

As noted, the sensor asymmetry (ΔT) is relatively insensitive topolarization and wavelength. In contrast, the asymmetry resulting fromthe overlay error is highly sensitive to polarization and wavelength.The factor K is very sensitive to polarization whereas ΔA is relativelyinsensitive to polarization. By using the sensitivity to polarization ofthe various components it is therefore possible to measure the overlayerror with a reduced influence of sensor asymmetry.

According to an embodiment of the invention there are two targets, afirst with a bias of +d and a second with a bias of −d. In anembodiment, the targets comprise a plurality of gratings but could takeany form. Both targets are illuminated with two orthogonal linearpolarizations, for example TE and TM radiation. There are therefore fourmeasured asymmetries:

A ₁₊ =K ₁(OV+d)+ΔA

A ¹⁻ =K ₁(OV−d)+ΔA

A ₂₊ =K ₂(OV+d)+ΔA

A ²⁻ =K ₂(OV−d)+ΔA

where subscripts 1 and 2 indicate the two polarizations and the + and −subscript indicate the sign of the bias d of the target. These fourasymmetries can be used by a processor, comprising, for example,suitable software, to calculate the overlay error OV with reducedinfluence of the sensor asymmetry:

${OV} = {d\frac{\left( {A_{2 +} - A_{2 -}} \right) - \left( {A_{1 +} - A_{1 -}} \right)}{\left( {A_{2 +} + A_{2 -}} \right) - \left( {A_{1 +} + A_{1 -}} \right)}}$

This method is most effective when the difference between K₂ and K₁ isas large as possible because the noise then has a less significanteffect. To reduce the effect of noise the overlay error could bemeasured for a plurality of targets on a substrate. This could be donefor the first substrate in a batch of substrates in order to calibratethe apparatus for subsequent exposure of substrates.

According to an embodiment of the invention, there are at least twotargets on a substrate, desirably positioned adjacent to each other asshown in FIG. 4. The detailed view in FIG. 5 of the target shown in FIG.4 shows that the first target 40 has a bias of +d and the second target50 has a bias of −d. Both of these targets 40, 50 are firstlyilluminated using TE radiation and the sensor asymmetry of the +1^(st)and −1^(st) orders for each of the targets calculated, A₁₊, A¹⁻. Thetargets are then illuminated using TM illumination and the sensorasymmetries, A₂₊, A²⁻ calculated.

The following equation is then used to calculate the overlay error:

${OV} = {d\frac{\left( {A_{2 +} - A_{2 -}} \right) - \left( {A_{1 +} - A_{1 -}} \right)}{\left( {A_{2 +} + A_{2 -}} \right) - \left( {A_{1 +} + A_{1 -}} \right)}}$

Although an embodiment of the invention is described in relation to the±1^(st) diffraction orders, an embodiment of the invention may beapplied using higher diffraction orders such as ±2^(nd), ±3^(rd),±4^(th), etc. diffraction orders.

Although an embodiment of this invention is described relating to two ormore different polarizations it could equally well be applied to two ormore different wavelengths because the overlay error is sensitive towavelength whereas the sensor asymmetry is relatively insensitive towavelength.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A method of measuring the overlay error of a substrate, comprising:projecting a first beam of radiation onto a first target of thesubstrate, the first target comprising at least two superimposedpatterns having a bias of +d between a first of the patterns arranged inor on a first layer and a second of the patterns arranged in or on asecond layer, and measuring the asymmetry of that first beam ofradiation reflected from the substrate that is indicative of a propertyof the substrate and generating a first signal indicative of themeasured asymmetry; projecting the first beam of radiation onto a secondtarget of the substrate, the second target comprising at least twosuperimposed patterns having a bias of −d between a first of thepatterns arranged in or on a first layer and a second of the patternsarranged in or on a second layer, and measuring the asymmetry of thatfirst beam of radiation reflected from the substrate that is indicativeof a property of the substrate and generating a second signal indicativeof the measured asymmetry; projecting a second beam of radiation ontothe first target and measuring the asymmetry of that second beam ofradiation reflected from the substrate that is indicative of a propertyof the substrate and generating a third signal indicative of themeasured asymmetry; projecting the second beam of radiation onto thesecond target and measuring the asymmetry of that second beam ofradiation reflected from the substrate that is indicative of a propertyof the substrate and generating a fourth signal indicative of themeasured asymmetry, wherein the first and second beams have differentpolarizations, or different wavelengths, or both, and the overlay erroris determined on the basis of the first, second, third and fourthsignals.
 2. The method of claim 1, wherein A₁₊ is the first signal, A¹⁻is the second signal, A₂₊ is the third signal and A²⁻ is the fourthsignal, the overlay error OV being given by:${OV} = {d\frac{\left( {A_{2 +} - A_{2 -}} \right) - \left( {A_{1 +} - A_{1 -}} \right)}{\left( {A_{2 +} + A_{2 -}} \right) - \left( {A_{1 +} + A_{1 -}} \right)}}$3. The method of claim 1, wherein the first and second beams are twosubstantially orthogonally linearly polarized beams.
 4. The method ofclaim 1, wherein the first and second targets are adjacent to each otheron the substrate.
 5. The method of claim 1, wherein the first and secondtargets form a pair of targets, the substrate comprising a plurality ofpairs of targets, each target of the pair having an equal and oppositebias to the other target of the pair and each pair located at adifferent position on the substrate, the method of claim 1 beingrepeated for each pair of targets to determine the overlay error at aplurality of different positions on the substrate.
 6. The method ofclaim 1, wherein the first and second targets each comprise a pluralityof gratings.
 7. The method of claim 6, wherein the first targetcomprises two gratings having a bias +d with respect to each other andthe second target comprises two gratings having a bias −d with respectto each other.
 8. The method of claim 1, wherein the first and secondbeams have substantially different wavelengths.
 9. A method ofmanufacturing a substrate comprising projecting a patterned beam ofradiation onto the substrate to expose the substrate wherein theexposing is based on overlay error determined by a method, comprising:projecting a first beam of radiation onto a first target of thesubstrate, the first target comprising at least two superimposedpatterns having a bias of +d between a first of the patterns arranged inor on a first layer and a second of the patterns arranged in or on asecond layer, and measuring the asymmetry of that first beam ofradiation reflected from the substrate that is indicative of a propertyof the substrate and generating a first signal indicative of themeasured asymmetry; projecting the first beam of radiation onto a secondtarget of the substrate, the second target comprising at least twosuperimposed patterns having a bias of −d between a first of thepatterns arranged in or on a first layer and a second of the patternsarranged in or on a second layer, and measuring the asymmetry of thatfirst beam of radiation reflected from the substrate that is indicativeof a property of the substrate and generating a second signal indicativeof the measured asymmetry; projecting a second beam of radiation ontothe first target and measuring the asymmetry of that second beam ofradiation reflected from the substrate that is indicative of a propertyof the substrate and generating a third signal indicative of themeasured asymmetry; projecting the second beam of radiation onto thesecond target and measuring the asymmetry of that second beam ofradiation reflected from the substrate that is indicative of a propertyof the substrate and generating a fourth signal indicative of themeasured asymmetry, wherein the first and second beams have differentpolarizations, or different wavelengths, or both, and the overlay erroris determined on the basis of the first, second, third and fourthsignals.
 10. An inspection apparatus configured to measure a property ofa substrate, the apparatus comprising: a radiation projector configuredto project radiation onto the substrate; a detector configured tomeasure asymmetry of radiation reflected from the substrate; and aprocessor configured to calculate an overlay error on the basis of theasymmetry, measured by the detector, of radiation of a plurality ofwavelengths, or a plurality of polarizations, or both, reflected fromthe substrate.
 11. The inspection apparatus of claim 10, wherein theradiation projector comprises a radiation source configured to supplythe radiation at a plurality of wavelengths, or at a plurality ofpolarizations, or both, onto the substrate.
 12. The inspection apparatusof claim 10, wherein the detector is configured to: measure theasymmetry of a first beam of radiation of first polarization,wavelength, or both, reflected from a first target of the substrate, thefirst target comprising at least two superimposed patterns having a biasof +d between a first of the patterns arranged in or on a first layerand a second of the patterns arranged in or on a second layer, andgenerate a first signal, measure the asymmetry of the first beam ofradiation reflected from a second target of the substrate, the secondtarget comprising at least two superimposed patterns having a bias of −dbetween a first of the patterns arranged in or on a first layer and asecond of the patterns arranged in or on a second layer, and generate asecond signal, measure asymmetry of a second beam of radiation of asecond polarization, wavelength, or both, different from that of thefirst beam, reflected from the first target and generate a third signal,and measure asymmetry of the second beam of radiation reflected from thesecond target and generate a fourth signal, and wherein the processor isconfigured to determine the overlay error on the basis of the first,second, third and fourth signals.
 13. The inspection apparatus of claim12, wherein A₁₊ is the first signal, A¹⁻ is the second signal, A₂₊ isthe third signal and A²⁻ is the fourth signal, and the processor isconfigured to determine the overlay error OV by:${OV} = {d\frac{\left( {A_{2 +} - A_{2 -}} \right) - \left( {A_{1 +} - A_{1 -}} \right)}{\left( {A_{2 +} + A_{2 -}} \right) - \left( {A_{1 +} + A_{1 -}} \right)}}$14. The inspection apparatus of claim 12, wherein the first and secondbeams are two substantially orthogonally linearly polarized beams. 15.The inspection apparatus of claim 12, wherein the first and second beamshave substantially different wavelengths.
 16. The inspection apparatusof claim 12, wherein the first and second targets are adjacent to eachother on the substrate.
 17. The inspection apparatus of claim 12,wherein the first and second targets each comprise a plurality ofgratings.
 18. The inspection apparatus of claim 17, wherein the firsttarget comprises two gratings having a bias +d with respect to eachother and the second target comprises two gratings having a bias −d withrespect to each other.
 19. The inspection apparatus of claim 12,configured to determine the overlay error at a plurality of differentpositions on the substrate by using a plurality of pairs of targets ofthe substrate, each pair of targets comprising the first and secondtargets and located at a different position on the substrate and eachtarget of the pair having an equal and opposite bias to the other targetof the pair.
 20. A lithographic apparatus, comprising: an illuminatorconfigured to condition a radiation beam; a support constructed to holda patterning device, the patterning device being capable of impartingthe radiation beam with a pattern in its cross-section to form apatterned radiation beam; a substrate table constructed to hold asubstrate; a projection system configured to project the patternedradiation beam onto a target portion of the substrate; and an inspectionapparatus configured to measure a property of a substrate, theinspection apparatus comprising: a radiation projector configured toproject radiation onto the substrate; a detector configured to measureasymmetry of radiation reflected from the substrate; and a processorconfigured to calculate an overlay error on the basis of the asymmetry,measured by the detector, of radiation of a plurality of wavelengths, ora plurality of polarizations, or both, reflected from the substrate.